1. Field of the Invention
The present invention relates to a capacitor which is employed in a MMIC (Monolithic Microwave Integrated Circuit), etc. in circuit portions, etc. of a radio device such as a portable telephone, a mobile phone, etc., for example.
2. Discussion of the Background
As shown in figures on pages 176, 177 of "Foundation of Microwave Circuit and its Application" (published by Sogo Denshi Publishing Co., Ltd., Feb. 1, 1992), for example, as for the conventional capacitor constructed on the MMIC, there are a MIM (Metal Insulator Metal) capacitor shown in FIG.8A as the first example and an inter-digital capacitor shown in FIG. 8B as the second example.
As shown in FIG. 8A, the MIM capacitor has a structure in which two conductors 1, 2 are stacked on a substrate 4 via a dielectric layer 3 to oppose to each other, and has such a feature that a large capacitance can be obtained by a relatively small pattern area. Since normally the MIM capacitor employed in the MMIC is formed by using a thin film process, a dielectric layer (e.g., SiO.sub.2) formed by the chemical vapor deposition is employed as the dielectric layer 3, or the dielectric layer 3 can be formed with resin by coating a polyimide resin paste on a conductor being formed on the substrate, or a ceramic dielectric layer can be formed by coating the dielectric paste on the conductor formed on the substrate by using the sol-gel method, etc. and then firing it. Since the dielectric layer connected by the above method can be formed to have a thickness of about several .mu.m, it is easy to implement the capacitor having a large capacitance with a small area.
As shown in FIG. 8B, the inter-digital capacitor as the second example in the prior art has a structure in which comb-type electrodes 5, 6 are opposed to each other on the same surface of the substrate 4. In other words, the comb-type electrodes 5, 6 have a plurality of element electrodes 7, 8 respectively, and the plurality of element electrodes 7, 8 are opposed to each other on a surface of the substrate 4 along the surface direction to form a capacitance. Normally, the inter-digital capacitor employed in the MMIC is formed by forming a conductive film on an overall surface of the substrate 4 by virtue of the sputtering, etc., then coating photoresist on the conductive film, then exposing and developing a pattern to be formed onto the photoresist, and then etching a conductive film portion to be removed. Hence, since both the comb-type electrodes 5, 6 of the inter-digital capacitor can be formed by the same step, they have a structure which has small variation of a capacitance value being accomplished in mass production especially.
According to the MIM capacitor shown in FIG. 8A as the first example in the prior art, the capacitance value is varied according to a film thickness of the dielectric layer 3 formed between the capacitor electrodes 1, 2. For example, in the case of the MIM capacitor which is formed to have the dielectric layer 3 of 5 .mu.m thickness, the capacitance value to be formed is subjected to the variation of .+-.10% even if a film thickness of the dielectric layer 3 can be formed with a precision of .+-.0.5 .mu.m. Though a precision of the capacitor depends on a precision of the filter circuit, etc., normally such precision of the capacitor employed in the filter circuit, etc. must be restrained in the range of about .+-.5% of the target value of the capacitance value, and the higher precision of the capacitor is also requested in some cases. Accordingly, in order to achieve such precision, a precision of the film thickness in forming the dielectric layer 3 must be suppressed less than .+-.0.25 .mu.m. However, in order to form the dielectric layer 3 within the foregoing precision in mass production, there are problems that the film thickness is readily varied if any above-mentioned methods are used to form the dielectric layer 3 and that especially the capacitance value to be formed is ready to vary as the dielectric layer 3 is made thinner.
According to the inter-digital capacitor shown in FIG. 8B as the second example in the prior art, the stable capacitance value can be derived in mass production as mentioned above, but it is hard to form the large capacitance value. Therefore, in order to get the large capacitance, the patterns of the comb-type electrodes 5, 6 must be increased in size, and thus they are unsuitable for the narrow pattern regions. In addition, as the method of increasing the capacitance value, the clearances between the element electrodes 7, 8 of the comb-type electrodes 5, 6 which are opposed on the surface of the substrate 4 must be designed small.
However, clearances between the element electrodes 7, 8 are formed by the etching, as described above. Therefore, the etching conditions are strictly restricted if such clearances are set extremely narrow, so that there is a problem such that variation in forming the electrodes is caused. More particularly, due to slight variation of the etching conditions in mass production, the conductive film cannot be sufficiently etched and thus short-circuit between the element electrodes 7, 8 is caused. Conversely, due to overetching, the element electrodes 7, 8 are formed too narrow and thus the element electrodes 7, 8 are eliminated in some areas.